Imaging elements have been devised which rely upon the photodestruction of o-phthalaldehyde which, where not destroyed, forms a dye when suitably developed. Examples are disclosed in U.S. Pat. No. 3,102,811 wherein poly(vinylpyrrolidone) and poly(vinyl alcohol) are listed as exemplary binders for an o-phthalaldehyde image-forming composition. o-Phthalaldehyde is also used as a dye-forming material in imaging elements which rely upon the reduction of cobalt(III) complexes, as described in Research Disclosure, Vol 158, June, 1977, Publication No 15874, published by Industrial Opportunities Ltd, Hampshire, United Kingdom.
Such imaging elements are susceptible to loss of phthalaldehyde during element formation, due to extreme volatility of the compound. Such losses can decrease drastically the amount of dye density available during development. One solution to this problem is to use as a binder for the phthalaldehyde a material which is adapted to retain the phthalaldehyde in the element during manufacturing. Particularly useful binders which provide superior levels of retention are described in commonly owned U.S. Application Ser. No. 971,464 by Fletcher et al filed on Dec. 20, 1978, entitled "Imaging Elements and Compositions Featuring Aromatic Dialdehyde-Retaining Binders."
Although the binders described in the aforesaid application greatly increase the available dye density in elements using phthalaldehyde as the dye-forming material, some phthalaldehyde can still be lost by volatilization during image processing. Losses particularly can occur when the exposed element is heated for image development. Accordingly, further retention of phthalaldehyde is desirable.
Various polymers have been used in the past as overcoats for a variety of imaging elements and for a variety of purposes. Common among such polymers are poly(vinyl pyrrolidone) and poly(vinyl alcohol) used separately, and those suggested in U.S. Pat. Nos. 3,984,253, Examples 5 and 9, and 4,075,019, layer 206 and Example 61. However, although some of these provide an increase in image densities when applied to a phthalaldehyde-imaging element, an increase greater than that obtainable with these has been sought.